The present invention relates to a capacitor using partially nitrided niobium as a lead wire material for electrodes.
Two electrodes and a dielectric material interposed therebetween constitute conventional capacitors. As one of such electrodes, there is used, for example, one selected from valve action metals such as tantalum, aluminum, niobium, titanium and alloys of these metals. To the electrode is connected one end of a lead wire for electrical connection with the outside. As for the material of lead wires, generally thin wires of tantalum and aluminum have been used.
In capacitors with armors in the form of final commercial products, one end of the lead wire is connected to the above-described one end of the electrode by, for example, welding, caulking, sintering after simultaneous molding or the like. The other end of the lead wire that is not connected to the electrode may be of the construction such that it is electrically connected, in the inside of the armor, to one end of an outer terminal separately provided for electrical connection from the inside of the armor to the outside. That is, it may be of the construction such that it is connected to a portion between the above-described one electrode and the outer terminal.
Further, for the above-described capacitors, those used in the inside of a recent electronic device are desired to be reduced in size, light in weight and have a large capacitance. On the other hand, there is formed also on a surface of a part of the lead wire connected to the above electrode, an oxide of the material of a takeout wire as a part of a dielectric layer provided between the electrodes. Since the connection parts between the electrodes and lead wires are of the structure that is unstable in stress, the dielectric layer formed near the connection parts containing a part of the lead wire is unstable in structure and stress. This is improved by increasing the thickness of the dielectric layer formed on the surfaces of a part of the lead wire and the electrode to which the lead wire is connected. However, the capacitors produced by this method usually tend to have a reduced capacitance, thus failing to meet to the requirement of large capacitance. This is because the thickness of the dielectric is increased. However, in the case where a lead wire composed of a niobium material is used, an increased capacitance is met if the thickness of the dielectric layer (niobium oxide) near the connection part of the lead wire is increased in order to avoid instability in structure and stress. This is because the niobium oxide has a large dielectric constant. Furthermore, since niobium is lighter in weight than tantalum, reduction in weight can be met simultaneously. However, the withstand voltage of capacitors produced using lead wires made of a niobium material is lower than that of conventional capacitors produced using lead wires made of tantalum and is unsatisfactory.
Accordingly, an object of the present invention is to provide a light capacitor having high withstand voltage using a lead wire made of niobium as a material without decreasing its capacitance.
Under the circumstances, the present inventors have made intensive research and as a result developed a lead wire made of a niobium material having a high withstand voltage with preventing a decrease in capacitance, thus achieving the present invention.
That is, the present invention has solved the above problems by proving the capacitors described in 1 to 5 below.
1. A capacitor comprising two electrodes and a dielectric interposed therebetween, wherein at least one of the electrodes is made of a valve action metal or its alloy and wherein a lead wire connected to the at least one electrode is made of partially nitrided niobium.
2. The capacitor as described in 1 above, wherein the partially nitrided niobium as the material of the lead wire has a nitrogen content of 10 to 150,000 mass ppm.
3. The capacitor as described in 2 above, wherein the partially nitrided niobium as the material of the lead wire has a nitrogen content of 100 to 100,000 mass ppm.
4. The capacitor as described in 3 above, wherein the partially nitrided niobium as the material of the lead wire has a nitrogen content of 500 to 7,000 mass ppm.
5. The capacitor as described in 1 above, wherein the electrode to which the partially nitrided lead wire is connected is made of at least one selected from the group consisting of tantalum, tantalum alloy, niobium and niobium alloy.
6. The capacitor as described in 1 above, wherein the electrode to which the partially nitrided lead wire is connected is made of at least one selected from the group consisting of tantalum, tantalum alloy, niobium and niobium alloy and is partially nitrided.
7. The capacitor as described in any of 1 to 4 above, wherein the electrode to which the partially nitrided lead wire is connected is of the structure obtained by integrally molding and sintering a partially nitrided niobium powder and powder of an electrode material and has a CV value of at least 50,000 (CV/g).
The reason why the capacitor of the present invention using a partially nitrided niobium as the material of the lead wire prevents the deterioration of withstand voltage has not been made clear but may be presumed as follows.
The above-described dielectric layer formed between the electrodes is formed also on a part of the lead wire connected to the electrode. However, if niobium is used as the material of lead wire, niobium tends to extract oxygen contained in the dielectric layer formed on the lead wire since niobium has higher affinity for oxygen than tantalum does. Therefore, the insulation of the dielectric layer formed near the connection part that includes a part of the lead wire decreases and as a result the withstand voltage decreases. It is considered that in the present invention, however, the niobium as the material of the lead wire is partially nitrided niobium, in which niobium is bonded to nitrogen so that the tendency that oxygen is extracted from the dielectric layer is alleviated to prevent the deterioration of the withstand voltage of a finally produced capacitor.